Embodiments of the invention relate to apparatuses and methods for gas-solid separation and particularly for the separation of gas effluents from fluidized particle beds, including those used for catalytic reactions and catalyst regenerations involving solid catalysts. Example embodiments relate more particularly to cyclone separators used in gas-solid separators.
Fluidized beds are currently used extensively in major industries including oil refining, petrochemical production, coal and mineral beneficiation, metallurgical applications, food processing, etc. Fluidized beds of solid particles, and particularly those operating in the bubbling regime, advantageously provide very uniform gas-solid contacting conditions due to thorough mixing. Fluidization generally causes not only local mixing but also large-scale circulation within the bed. These benefits of solid particle fluidization, however, are not without consequences. The most significant of these is the entrainment (elutriation or carryover) of solid particles due to the passage of gas bubbles through the dense phase of the fluidized solid particle bed and breakage of these bubbles at the surface of the dense phase. The bursting action of the bubbles throws large amounts of the particulate solids into the dilute phase directly above the dense phase. This in turn causes entrainment of particles having a sufficiently small diameter, namely such that their terminal velocity (which decreases with decreasing particle size) is below the superficial velocity of the rising gas.
Particular fluidized bed systems of practical interest in the refining and petrochemical fields include those used in catalytic conversions in the presence of a solid particulate catalyst. The use of fluidized beds of catalyst is favorable, for example, in conversion processes in which catalyst deactivation, due to the accumulation of carbonaceous deposits (coke) during the course of the conversion, occurs rapidly. In such cases, deactivated catalyst from a reaction zone must be passed to a regeneration zone for removal of the accumulated deposits by combustion, followed by return of the regeneration catalyst back to the reaction zone. Fluidized beds of catalyst in both the catalytic reactor and catalytic regenerator allow for continuous circulation of spent (coked) and regenerated catalyst between these apparatuses.
One example of a refining process utilizing fluidized bed reaction and regeneration zones is fluid catalytic cracking (FCC). FCC is applicable for the conversion of relatively high boiling or heavy hydrocarbon fractions, such as crude oil atmospheric and vacuum column residues and gas oils, to produce more valuable, lighter hydrocarbons and particularly those in the gasoline boiling range. The high boiling feedstock is contacted in one or more reaction zones with a particulate cracking catalyst that is maintained in a fluidized state under conditions suitable for carrying out the desired cracking reactions. In the fluidized contacting or reaction zone, carbonaceous and other fouling materials are deposited on the solid catalyst as coke, which reduces catalyst activity. The catalyst is therefore normally conveyed continuously to another section, namely a regeneration zone, where the coke is removed by combustion with an oxygen-containing regeneration gas. The resulting regenerated catalyst is, in turn, continuously withdrawn and reintroduced in whole or in part to the reaction zone.
More recently, fluidized bed systems have been applied in the production of light olefins, particularly ethylene and propylene, which are valuable precursors for polymer production. The light olefins are desirably obtained from non-petroleum feeds comprising oxygenates such as alcohols and, more particularly, methanol, ethanol, and higher alcohols or their derivatives. Methanol, in particular, is useful in a methanol-to-olefin (MTO) conversion process described, for example, in U.S. Pat. No. 5,914,433. This patent and others teach the use of a fluidized bed reactor with continuous circulation of spent catalyst from the reactor to a regenerator. The regenerator can similarly contain a fluidized bed of solid catalyst particles for carrying out regeneration by the combustion of deposited coke.
In processes such as FCC and MTO, the use of fluidized particle beds in the reaction and regeneration zones leads to entrainment of solids into the gaseous effluents from these zones. In the case of the reaction zone, catalyst particles can exit with the reactor effluent, containing the desired reaction products (e.g., gasoline boiling range hydrocarbons in the case of FCC or light olefins in the case of MTO). Likewise, catalyst particles may similarly become entrained in the combustion gases exiting the catalyst regenerator (e.g., containing nitrogen, CO2, CO, and H2O). Catalyst fines contained in the regenerator flue gas effluent are known to interfere with downstream power generation equipment such as the expander. In general, the losses of entrained catalyst from a fluidized bed, such as a catalytic reactor or catalyst regenerator, result in increased costs, particularly on an industrial scale. This is especially true considering the high cost of the zeolite-containing catalysts used currently in FCC and the non-zeolitic molecular sieve catalysts (e.g., silicoaluminophosphates or SAPOs) used currently in MTO.
To minimize losses of entrained catalyst particles, a number of gas-solid separators have been proposed for use in disengagement or separation zones, located above the dense bed phase, in reactors and regenerators having fluidized solid catalyst beds. These separators, including cyclones, filters, screens, impingement devices, plates, cones, and other equipment, have been used with varying success. Cyclone separators have gained widespread use in both FCC and MTO as described, for example, in U.S. Pat. No. 8,419,835 and in U.S. Pat. No. 6,166,282. Cyclone separators have been applied in both the catalytic reactors and catalyst regenerators of these conversion processes.
Refiners have also used a cyclone-containing third stage separator (TSS), external to the catalyst regenerator, to remove catalyst fines from the FCC regenerator flue gas (i.e., the combustion gas exiting the regenerator). These devices have typically been used in power recovery installations to protect expander blades. In the TSS, flue gas from the FCC catalyst regenerator is passed through a number of high efficiency cyclonic elements arranged in parallel and contained within the TSS vessel. The flue gas enters the vessel through a flow distributor that evenly distributes the gas to the individual cyclone elements. After catalyst particulates are separated from the flue gas in the cyclones, the clean flue gas leaves the separator. The solid particulates are concentrated in a small stream of gas, called the underflow gas, which exits the bottom of the TSS.
Cyclones and other separation devices exhibit equipment (e.g., metal) erosion due to the high velocity of gases used and the interaction between these gases, containing entrained solid particles, and the walls of these devices. Erosion leads to a reduction in equipment life and/or increased costs due to maintenance and downtime. For example, it has been observed that, over the course of continuous operation over a prolonged time period on the order of several years, considerable erosion may occur within the separator cyclone barrel. If sufficiently severe, such erosion may require localized repair or replacement of the entire cyclonic separator, which may necessitate shutdown of the TSS and possibly the FCC system as a whole.
Erosion issues mainly arise from a large central hub outside diameter within the cyclone. Typically, swirling flow in an annular section within the cyclone (between the central hub and the cyclone barrel) contracts from a diameter corresponding to the central hub outside diameter to the center of the cyclone barrel, with an associated increase in the velocity and a low pressure region at the center. As a result, the gas stream at the barrel's inner diameter (i.e., the barrel wall) momentarily contracts to a smaller diameter. This causes an abrupt change in the particle flow direction and erodes the barrel wall. In addition to the erosion problem, the large empty volume taken up by the central hub is inefficient and increases the size of the vessel housing the cyclones.
Other devices are designed to circumvent the erosion problem by simply reducing the central hub diameter. However, for a given helical pitch (turns/angle), the path traced by the helical curve is more vertical at the hub and more horizontal at the cyclone's barrel. For this reason, the flow at the center will have a more vertical trajectory than at the outside, which is undesirable from a separation perspective. This disadvantage can be circumvented by varying the height of the swirl vane along the radius, for instance by using a small height at the central hub and a larger height at the barrel. However, this creates a potential for structural vibration issues due to the large span.
There is therefore an ongoing need in the art for apparatuses and methods that promote the desired separation of solids (e.g., catalyst particles), from gases (e.g., reactor and regenerator effluents) into which these solid particles are entrained, while simultaneously minimizing the erosion and consequent particle attrition.